Signal separating system

ABSTRACT

A signal separating system  1  comprises a frequency conversion element  3 , a dynamic filter component  4  with associated coherent stop slot and non-coherent passband, optionally a signal wave enhancer  7  with bypass, and a frequency reconversion element  8.  In operation of the system, mixed incoming signals  12  (CW or pulse signals) having known interference and unknown emission components can be effectively separated, permitting a virtually instantaneous separation of the known interference from the unknown emissions and permitting an unobscured monitoring and/or characterisation of the extracted unknown emissions  21,  if desired. The arrangement bears definite advantage over known arrangements and retains utility for various frequency/agile or multiple or non-agile unknown signal applications.

FIELD OF THE INVENTION

[0001] The present invention relates to a signal separating system andmore particularly, but not exclusively, concerns a signal separatingmethod and apparatus for separating known transmission interference fromunknown radar signal emissions.

BACKGROUND OF THE INVENTION

[0002] Monitoring unknown pulsed emissions or continuous wave (CW)emissions is difficult in the presence of high level CW emissions in themonitored bandwidth. Pulsed emission monitoring in the presence ofpulsed interference can be overcome using established pulse on pulseextraction techniques but CW high level interference tends to maskunknown low level pulses within the monitored bandwidth. In the past,static or adaptive filters have been used to remove CW interference,creating blind spots in the monitored bandwidth. Further, adaptivefilters, for example adaptive YIG filters, have settling times which canbe too slow to remove fast agile signal emissions such as very fastfrequency hopped CW wideband communications emissions and are notreadily adaptable to remove multiple CW spread spectrum interferingemissions.

OBJECTS AND SUMMARY OF THE INVENTION

[0003] The present invention aims to overcome or at least substantiallyreduce some of the above-mentioned drawbacks.

[0004] It is an object of the present invention to provide an improvedsignal separating system which has a virtually zero settling timeresponse, which is unaffected by multipath effects/amplitude variationsand which has the capability of instantly separating known transmissioninterference from unknown signal emissions, for example radar signalemissions, and of monitoring and optionally enhancing the unknown signalemission characteristics within the same bandwidth as the known ownemissions without interference.

[0005] It is another object of the present invention to provide a signalseparating system with novel real-time frequency agile filter means,enabling an enhancement in the system's capability of measuring/sensingreceived signal emissions.

[0006] In broad terms, the present invention resides in the concept ofusing instantaneous frequency agile adaptive conversion as means tofrequency convert an incident mixed signal and controllably transmittingthrough a predetermined filter region an incoherent signal component ofthe frequency converted signal representative of an unknown signalemission and instantaneously reconverting the extracted unknown signalemission to its original frequency, enabling the unknown signal emissioncharacteristics to be measured and monitored without the unwantedtransmission interference effects.

[0007] According to one aspect of the present invention, there isprovided a method of separating an unknown signal emission from a knowninterference signal in a system incorporating frequency filter meanswith controllable frequency converting means, the method comprising thesteps of:

[0008] (a) receiving and frequency converting a mixed signal at thefrequency converting means, which mixed signal comprises a first signalcomponent representative of the unknown signal emission and a secondsignal component representative of the known interference signal, thefirst signal component having a single frequency or a plurality offrequencies associated herewith;

[0009] (b) stopping a coherent signal component representative of thesecond signal component at a selected frequency stopband regionassociated with the frequency filter means;

[0010] (c) transmitting an incoherent signal component representative ofthe received first signal component through a selectedfrequency-transparent region associated with the frequency filter means;and

[0011] (d) reconverting the transmitted incoherent signal component withreference to a predetermined signal so as to restore the unknown signalemission at its said single frequency/said plurality of frequencies.

[0012] According to another aspect of the present invention, there isprovided a signal separating system for separating an unknown signalemission from a known interference signal, the system comprising:

[0013] means for receiving and frequency converting a mixed signal,which mixed signal comprises a first signal component representative ofthe unknown signal emission and a second signal component representativeof the known interference signal, the first signal component having asingle frequency or a plurality of frequencies associated herewith;

[0014] means for stopping a coherent signal component representative ofthe second signal component at a selected frequency stopband region ofthe system;

[0015] means for transmitting an incoherent signal componentrepresentative of the first signal component through a selectedfrequency-transparent region of the system; and

[0016] means for reconverting the transmitted incoherent signalcomponent with reference to a predetermined signal so as to restore theunknown signal emission at its said single frequency/said plurality offrequencies.

[0017] In accordance with several exemplary embodiments of the inventionwhich will be described hereinafter in detail, the signal separatingsystem uses instantaneous agile adaptive conversion to diffuseincoherent signal emissions through transparent regions of a staticfilter and removes coherent emissions directed into a static narrow stopslot, having virtually no settling time and instantly separating knownunwanted interference effects from unknown (for example, radar) signalemissions. Further, by removing known coherent emissions, diffusedunknown emissions can be separated and instantaneously reconverted totheir original frequency, allowing the unknown signal emissionscharacteristics to be monitored effectively without interference andproviding a proper signal characterisation if desired, (aPW/PRF/frequency characterisation for example).

[0018] Advantageously, the provision of instantaneous agile frequencyconversion allows

[0019] (a) signal rejection to be performed at a particular frequencyenabling narrow coherent frequency slots to be deployed, and

[0020] (b) signal transmission to be effected at a particular frequencyfor signal to noise enhancement, if required. This increases measurementsensitivity of the signal separating system of the invention, typicallyby 30 dB for low level pulse monitoring.

[0021] Advantageously the incident mixed signal can be effectivelyseparated at a number of different frequencies using a number of agilefrequency converters. The mixed signal may comprise a continuous orpulsed wave signal, for example a signal in the radio frequency range.

[0022] Conveniently, the signal to noise level of the transmittedincoherent signal component can be enhanced. For example, the dynamicrange of the transmitted incoherent signal component can be enhanced bymeans of signal compression or signal logarithmic amplification.Further, the transmitted incoherent signal component can be enhanced ata predetermined frequency by use of a magnetostatic wave (MSW) enhancer,permitting the threshold sensitivity of the system to beoptimised/enhanced and permitting the suppression of unwanted noise andspurii.

[0023] To achieve signal enhancement using a magnetostatic wave enhancer(MSW), it is envisaged that a Gadolinium Gallium Garnet substrate deviceis used in the system of the invention. Alternatively, any device havingsimilar characteristics to that of Gadolinium Gallium Garnet substratecould be deployed for MSW signal enhancement in the invention.

[0024] It is noted that the signal to noise enhancement can beadvantageously carried out at different frequencies using additionalfrequency converters, if desired.

[0025] Conveniently, the signal separating system of the invention canbe used to monitor both narrow pulses (100 ns pulsewidth typically) andbroad pulses in the presence of CW emissions typically greater that 40dB above the pulse levels, without creating blind spots for agile CWinterference. Also, the components of the signal separating system canbe deployed in parallel or serial combinations to remove multiple CWinterference from known emitters, thus permitting signal monitoring inconditions not readily achievable previously without blind spots.

[0026] It is appreciated that the signal separating system of theinvention finds utility in multiple, in frequency agile and in non-agileunknown signal applications. In frequency agile applications, forexample, the frequency stopband region can be conveniently modified topermit stopping of the coherent signal component at a particular(normally low) frequency. Preferably, the frequency filter component ismaintained stationary, enabling the frequency filtering of signals to beinstantaneous with virtually zero settling time.

[0027] Having regard to the foregoing, the signal separating system ofthe invention has enhanced performance over known systems and it canprovide interference rejection typically greater than 70 dB and signalenhancement typically up to a 30 dB level. Instantaneous reception ofunknown pulses at levels typically down to −90 dBm with 100 nspulsewidths can be achieved in the presence of input communicationinterference levels greater than 40 dB, typically, above the pulse inthe same band. Also, the system of the invention can (a) provideradiation monitoring in the same bandwidth without frequency obstructionfor FHSS (agile) communications interference and (b) reduce or minimisethe frequency obstruction when deployed with non-agile communications.

[0028] Further the system of the invention can conveniently provide forunknown signal monitoring without the use of special known signalwaveforms which could otherwise indicate the known signal emitter'stactical role and identity.

[0029] Further, signal separation in the system of the invention can beeffectively carried out with pulse or CW unknown and known signalcombinations using modulated or unmodulated signals.

[0030] The above and further features of the invention are set forthwith particularity in the appended claims and will be describedhereinafter with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a block schematic of a signal separating apparatusembodying the present invention;

[0032]FIG. 2 to 4 are schematics of a signal separation processembodying the invention for fixed frequency unknown signals and formultiple or agile unknown signals;

[0033]FIG. 5 is a schematic of a cascaded filter arrangement in anembodiment of the invention;

[0034]FIG. 6(a) shows an external view and an internal view of amagnetostatic wave (MSW) substrate device enhancer for use in theinvention;

[0035]FIG. 6(b) shows typical signal enhancement characteristics of thedevice enhancer of FIG. 6(a);

[0036]FIG. 7 shows typical signal reception characteristics of a signalseparating system embodying the invention;

[0037] FIGS. 8(a) and 8(b) show some further typical signal receptioncharacteristics of a signal separating system embodying the invention;and

[0038]FIG. 9 shows typical spectra characteristics of a signalseparating system embodying the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0039] In this specification, the term “frequency agile” is taken tomean or cover any form of signal which can move across the frequencyband by changing its frequency.

[0040] Referring first to FIG. 1, there is schematically shown apreferred signal separating apparatus 1 embodying the present invention.The signal separating apparatus 1 comprises an agile frequencyconversion element 3, a frequency filter 4 with an associated coherentfrequency stop slot 5 and non-coherent frequency passband 5, amagnetostatic wave (MSW) enhancer 7 with bypass and a frequencyreconversion element 8.

[0041] As shown in FIG. 1, the agile frequency conversion element 3receives and combines at is input side (i) an incident mixed signal 12comprising known and unknown signal components and (ii) a known controlreference signal 14. The input signal is frequency converted by theconversion element 3 and the resultant frequency converted signal 18 isoutput to the connecting frequency filter 4. The frequency filter 4 hasa high rejection narrowband coherent stop slot (static) 5 and anon-coherent frequency passband 5, so that the known coherent part ofthe frequency converted signal is blocked by the stop slot 5 at aselected frequency, and the unknown incoherent part of the frequencyconverted signal is transmitted and diffused through the selectedfrequency passband 5. The diffused incoherent (unknown) signal 19 ispreferably received by the connecting magnetostatic wave (MSW) enhancer7, with bypass, for providing signal to noise enhancement and theresultant enhanced (unknown) signal 20, together with the known controlreference signal 14, are controllably fed into the input side of theconnecting frequency reconversion element 8 so that the unknown signalis frequency converted by the reconversion element 8 back to itsoriginal incident frequency. The frequency reconversion step is effectedby performing a comparison between the unknown signal characteristic 20and the known control reference signal 14.

[0042] Thereafter, as shown in the Figure, the restored unknown signalemission 21 can be effectively monitored in real time by an externalmonitor system 10 if desired, permitting an in-depth characterisation ofthe unknown signal emission 21 if required.

[0043] In operation of the thus described signal separating apparatus,it will be understood that the MSW enhancement stage is optional, thisbeing optionally deployed at a selected frequency using a GadoliniumGallium Garnet substrate device or like device having similarcharacteristics. Also, while the components in the described embodimentare showing to be separate units, it is possible that the componentscould be formed as an integral unit.

[0044] Referring now to FIG. 2, this shows in schematic form the variousstages of the known/unknown signal separation process according to oneembodiment of the present invention as applied to incident mixed signalshaving a fixed frequency unknown signal component.

[0045] In FIG. 2, there is shown an incident mixed signal for use in theinvention which has a fixed frequency unknown signal component 22 and aspread frequency known signal component 22′ (stage 1). The mixed signalis frequency converted and the result is to (i) spread the unknownsignal component into a number of unknown signals 24 over a range offrequencies and (ii) despread the known signal component so as toproduce a known coherent signal 25 within a narrow frequency band (stage2). As shown, the resultant signal is subject to a dynamic frequencyfilter 26 (stage 3), the known coherent signal being blocked by theselected stopband region 27 of the filter and the unknown signals beingallowed to pass/diffuse through the transparent frequency bands 28 ofthe filter. The diffused incoherent unknown signals 30 can be(optionally) signal enhanced (stage 4) and thereafter, as shown, theunknown signals are frequency reconverted back to their originalincident frequency 31 (stage 5). It is therefore an important feature ofthe present invention that the frequency reconversion provides a way ofreversing the original frequency conversion by restoring unknowndespread signals to their original frequency. As shown, the resultantextracted unknown signal 31 can be monitored in real time by a monitorsystem (stage 6).

[0046] Referring to FIG. 3, this shows in schematic form the variousstages of the known/unknown signal separation process according toanother embodiment of the invention as applied to incident mixed signalshaving multiple or agile frequency unknown signal components. In thefollowing description, however, only the differences between the processof FIG. 3 and the previously described process of FIG. 2 are described.

[0047] In FIG. 3, the incident mixed signal has multiple or agilefrequency unknown signal components 35 and a spread frequency knownsignal component 36 (stage 1). The mixed signal is frequency converted,filtered and enhanced (stages 2 to 4) and the result is the same asdescribed previously in the FIG. 2 embodiment. As shown, the resultantspread (enhanced) unknown signals 37 are frequency reconverted, enablingthe multiple or agile frequency unknown signals to be restored to theiroriginal frequency 38 (stage 5) and to be monitored if desired (stage6).

[0048]FIG. 4 shows how particular frequency spectra of an incidentknown/unknown signal, which is to be separated, are formed during theabove described separation process and how there is a correspondencebetween the generated spectra and the various stages of the abovedescribed signal separation process.

[0049] More particularly, as shown in frequency spectrum A of FIG. 4, anincident single carrier signal with a low rf frequency is received by anaerial and the received signal is frequency converted by an adaptiveconverter to produce a spread unknown signal over a range of frequenciesand a despread known coherent signal within a specifically narrow lowfrequency band. As shown, the adaptive converter is controlled from acoherent sample which provides a known reference signal, and, in turn,this mode of signal control is used to direct the known coherent owntransmissions into the fixed low frequency narrowband stop region of thefrequency filter and to diffuse the spread incoherent unknown signalsthrough the transparent frequency bands of the frequency filter. Asshown, the resultant signal is preferably frequency reconverted via atwo-stage reconversion process, there being incorporated an additionalintermediate wideband signal to noise enhancement stage to reduce theeffects of unwanted noise and spurii in the invention. The result is toprovide an extracted unknown signal only at its original frequency.

[0050] Further, as shown in frequency spectrum B of FIG. 4, an incidentsingle carrier signal with high rf frequency is shown, the onlydifference between the signal spectrum B and the signal spectrum A, aspreviously described, being that the location of the filter stop band ismodified so as to be at a predetermined high rf frequency rather than alow rf frequency, enabling the known coherent signal component to bestopped at a predetermined high, rather than low, frequency.

[0051]FIG. 5 illustrates how a cascaded filter arrangement 50 of simpledesign is preferably used in the signal separating system of theinvention in order to remove extraneous known signals 52, 52′multi-carrier known signals for example, from the system.

[0052] In FIG. 5, there is shown an incident mixed signal 51,52,52′comprising a spread unknown signal 51 and, by way of example, twoextraneous despread known signals 52 52′ having first and secondfrequencies. As shown, the incident signal 51,52,52′ is subject to acascaded, serial-type filter arrangement 50 comprising a first frequencyfilter 55 with an associated selected narrowband stop 56 at said firstfrequency and a second frequency filter 57 with an associated selectednarrowband stop 58 at said second frequency. As is also shown, knowncontrol reference signals 59,60 are fed into the first and secondfrequency filters. It is to be appreciated that the number of filters tobe used in the cascaded arrangement 50 could be easily varied independence upon the variable number of identified extraneous knownsignals, and that a simple cascaded parallel-type arrangement could bealternatively used instead of the above described serial-typearrangement.

[0053] In operation of the cascaded serial-type arrangement, therefore,the incident signal 51, 52, 52′ is frequency converted (not shown inFIG. 5) and thereafter, the resultant converted signal is subject to thefirst frequency filter 55 so that the known signal 52 having the firstfrequency is blocked by the first filter stop 56 and the remaining partof the signal is allowed to pass through the transparent frequency bands63 of the filter. As shown, the transmitted signal is then subject tothe second frequency filter 57 so that the remaining known signal 52′having the second frequency is blocked by the second filter stop 58 andthe unknown signal 61 is allowed to pass through the filter'stransparent frequency band 62 for subsequent signal enhancement,frequency reconversion and, if desired, monitoring.

[0054]FIG. 6(a) shows an external view and an internal view of amagnetostatic wave (MSW) substrate device enhancer 70 for use in thesystem of the invention. The device enhancer 70, which is well-suited tovarious signal to noise enhancement applications, is designed to operatein a magnetic field.

[0055] As previously described, the signal separation process of theinvention, as applied to both frequency/agile and non-agile unknownsignals, optionally entails the deployment of signal to noiseenhancement at an optimum/selected frequency using an MSW deviceenhancer of the kind shown in the Figure, enabling the thresholdsensitivity of the system of the invention to be enhanced or optimised.

[0056] Preferably, the MSW device(s) to be used in the invention areformed of Gadolinium Gallium Garnet substrate material, although anydevice material with similar characteristics could be alternativelydeployed. Also, it is to be noted that wideband signal to noiseenhancement (over 0.6 to 1 GHz frequency range, typically) is madepossible in the system of the invention by use of the MSW device(s),enabling narrow signal pulses to be transmitted and reducing adaptiveconverter spurii and noise such as to enhance signal sensitivity of thesystem.

[0057] Conveniently, signal separation and signal enhancement in theinvention can be effected at a number of different selected frequenciesusing a combination of frequency converters and MSW device enhancers.

[0058]FIG. 6(b) shows some typical signal to noise enhancementcharacteristics of the MSW device enhancer of FIG. 6(a). As shown, thesignal enhancement level progressively increases, typically, as theinput rf power to the MSW device is allowed to increase over the range 0to 25 dBm and it is to be noted that the signal to noise enhancementlevel typically reaches a saturation level at around 32 dB correspondingto an input rf power of around 25 dBm.

[0059]FIG. 7 illustrates typical signal reception characteristics of asignal separating system embodying the present invention. As shown, anunknown CW signal of −70 dBm with CW known interference of −45 dBm inthe same received bandwidth is received by the system and thereafter, inaccordance with the above described separation process of the invention,the known own signal is cancelled and the desired unknown signalcomponent is extracted and signal enhanced. Note that both CW and pulsedunknown/known signal combinations, using modulated or unmodulatedsignals, can be effectively separated using the system of the invention.

[0060] FIGS. 8(a) and 8(b) illustrate some more typical signal receptioncharacteristics of a signal separating system embodying the presentinvention. In the figures, there is shown a −70 dBm pulse radar signalwith (a) 0.1 micro seconds pulsewidth (no signal to noise enhancement)and (b) 0.5 microseconds pulsewidth (no signal to noise enhancement),the signals being extracted from incident known/unknown signals inaccordance with the signal separating process of the invention, aftercancellation of a −45 dBm known interference signal (not shown) withinthe same receiver bandwidth.

[0061]FIG. 9 illustrates some typical spectra characteristics of asignal separating system embodying the present invention. In the Figure,there is shown a pulse spectrum 85 representative of an extracted −70dBm radar pulse signal (signal enhanced) which has been separated froman incident known/unknown signal in accordance with the signalseparating process of the invention, after cancellation of a −45 dBmFHSS (Frequency hop spread spectrum) known interference CW communicationsignal 80 occupying the same bandwidth as the radar signal. The level ofthe noise/spurii background 86 is shown. As shown, the known CW spectrum80 (dashed line) representative of the cancelled signal exceeds thepulse level by 25 dB (Figure not to scale) and it is to be noted thatthe known spectrum 80 is superimposed onto the extracted radar pulsespectrum 85 for the sake of comparison.

[0062] Having thus described the present invention by reference tovarious embodiments, it is to be well understood that the embodiments inquestion are exemplary only and that modifications and variations aswill occur to those possessed of the appropriate knowledge and skillsmay be made without departure from the spirit and scope of the inventionas set forth in the appended claims and equivalents thereof. Forexample, different kinds of signal having different shapes/waveforms,amplitudes and/or bandwidths can be received and separated by the systemof the invention. Note that pulse or CW unknown and known signalcombinations using modulated or unmodulated signals can be separated. Inaddition, the above described signal enhancement stage is optional in sofar as it is not an essential feature to the invention and it is to benoted that the system of the invention does not exclude the use ofsignal compression, signal logarithmic amplification and/or signallimiting to provide an enhancement of the signal dynamic (power) range,if desired. Further, the respective various stages in the system of theinvention, as described, can be varied in their order and/or number toprovide the same or similar technical effect; for example, the extractedunknown signal could be frequency reconverted via a multistage frequencyreconversion process with or without signal to noise enhancement, ifdesired. Also, additional frequency converters could be deployed in thesystem to provide filtering and/or signal enhancement at differentfrequencies. Further, whilst in one of the described embodiments, acascaded two filter serial arrangement is used for removing extraneousknown signals, it is to be noted that the number of filters could beeasily varied and that a cascaded parallel arrangement could equally beused in place of the described serial arrangement, if desired.

[0063] It is to be appreciated that the system of the invention findsutility in various frequency/agile or multiple unknown signalapplications, as described, as well as in non-agile unknown signalapplications. The above described system could be used in various radarcommunication systems. The system could be used in naval radarcommunication systems for example, to provide an interference freeinterface without radar signal masking. The system could also be used inECM (electronic counter measures) and/or ESM (electronic supportmeasures) applications.

[0064] Abbreviations

[0065] BP Bandpass

[0066] CW Continuous Wave

[0067] ECM Electronic Counter Measures

[0068] ESM Electronic Support Measures

[0069] (F) FHSS (Fast) Frequency Hopping Spread Spectrum

[0070] LP Low Pass

[0071] MSW Magnetostatic Wave

[0072] PRF Pulse Recurrent Frequency

[0073] PW Pulse Width

[0074] RF Radio Frequency

[0075] RX Receiver

[0076] S/N Signal to Noise Ratio

[0077] SNE Signal to Noise Enhancer

[0078] TX Transmitter

[0079] YIG Yttrium Iron Garnet

1. A method of separating an unknown signal emission from a knowninterference signal in a system incorporating frequency filter meanswith controllable frequency converting means, the method comprising thesteps of: (a) receiving and frequency converting a mixed signal at thefrequency converting means, which mixed signal comprises a first signalcomponent representative of the unknown signal emission and a secondsignal component representative of the known interference signal, thefirst signal component having a single frequency or a plurality offrequencies associated herewith; (b) stopping a coherent signalcomponent representative of the second signal component at a selectedfrequency stopband region associated with the frequency filter means;(c) transmitting an incoherent signal component representative of thereceived first signal component through a selected frequency-transparentregion associated with the frequency filter means; and (d) reconvertingthe transmitted incoherent signal component with reference to apredetermined signal so as to restore the unknown signal emission at itssaid single frequency/said plurality of frequencies.
 2. A method asclaimed in claim 1, further comprising enhancing the transmittedincoherent signal component to enhance the signal to noise level of thefirst signal component.
 3. A method as claimed in claim 2, wherein thedynamic range of the transmitted incoherent signal component is enhancedby means of signal compression.
 4. A method as claimed in claim 2,wherein the dynamic range of the transmitted incoherent signal componentis enhanced by means of logarithmic amplification.
 5. A method asclaimed in claim 2, wherein the transmitted incoherent signal componentis enhanced at a predetermined frequency by means of a magnetostaticwave enchancer.
 6. A method as claimed in claim 1, wherein the selectedfrequency stopband region is modified to permit the coherent signalcomponent to be stopped at a pre-determined frequency, enabling thestopband width to be reduced.
 7. A method as claimed in claim 1, whereinthe mixed signal comprises a radio-frequency continuous wave signal. 8.A method as claimed in claim 1 wherein the mixed signal comprises aradio-frequency pulsed wave signal.
 9. A method as claimed in claim 1wherein the signal separation and/or the signal enhancement are effectedat a number of different frequencies by means of frequency conversion.10. A method as claimed in claim 1, wherein the restored unknown signalemission is monitored in real time to permit a signal characterizationin dependence thereon.
 11. A signal separating system adapted andarranged to carry out a method as claimed in claim
 1. 12. A signalseparating system for separating an unknown signal emission from a knowninterference signal, the system comprising: means for receiving andfrequency converting a mixed signal, which mixed signal comprises afirst signal component representative of the unknown signal emission anda second signal component representative of the known interferencesignal, the first signal component having a single frequency or aplurality of frequencies associated herewith; means for stopping acoherent signal component representative of the second signal componentat a selected frequency stopband region of the system; means fortransmitting an incoherent signal component representative of the firstsignal component through a selected frequency-transparent region of thesystem; and means for reconverting the transmitted incoherent signalcomponent with reference to a predetermined signal so as to restore theunknown signal emission at its said single frequency/said plurality offrequencies.
 13. A signal separating system as claimed in claim 12further comprising signal enhancement means to enhance the transmittedincoherent signal component.
 14. A signal separating system as claimedin claim 13 wherein the signal enhancement means comprises amagnetostatic wave enhancer.
 15. A signal separating system as claimedin claim 12 incorporating a Gadolinium Gallium Garnet substrate device.16. A signal separating system as claimed in claim 12, including anumber of components arranged in parallel or serial cascades so as toremove extraneous known signals, multi-carrier known signals forexample, from the system.
 17. A signal separating system as claimed inclaim 12, adapted and arranged for application to frequency-agile,multiple and/or non-agile unknown signals.
 18. A radar systemincorporating a signal separating system as claimed in claim
 12. 19.(Cancelled)